For some decades a continuously variable transmission (CVT) has been used, generally in an experimental sense, to provide a continuously variable drive ratio between an engine and a load. The CVT has been successfully tested in an automotive vehicle, to provide an appropriate ratio between the engine and the rear wheels, rather than being limited by discrete step changes of ratio of the type provided by conventional gear trains. Various types of control systems for CVT's have been developed and implemented.
One example of such a control system is described in U.S. Pat. No. 4,152,947, which issued May 8, 1979. This patent discloses the use of one valve to control the belt tension between the adjustable pulleys, and a second valve to regulate the effective pulley diameters, thus to control the transmission ratio. Like other control systems developed for CVT's, the identified system suffers from various shortcomings. Among these are the production of a line pressure higher than required when the system is operating at normal load speeds with low torque demand. In addition the disclosed follower system for translating movement of one adjustable sheave into a fluid signal imposes a load on that sheave greater than desired. Another area needing improvement is the operation of the system ratio control valve.
It is thus a primary consideration of the present invention to provide an improved control system for a CVT, with a line pressure substantially lower than that provided in earlier systems when the operating conditions permit. A corollary consideration is the production of an improved throttle valve and associated components to achieve this lower line pressure in the system.
Another significant consideration is the provision of such a system with an improved ratio valve, that is, the valve which senses the position of one movable sheave and provides a fluid signal which varies as a function of transmission ratio.
An important consideration is the provision of a control system with significantly improved operation of the ratio control arrangement.